Optical glass, preform for precision press molding, optical element and method of manufacturing thereof

ABSTRACT

An optical glass having a refractive index n d  of 1.70 or greater and an Abbé number of 50 or greater. Given as mole percentages, it comprises: B 2 O 3  (20 to 80 percent), SiO 2  (0 to 30 percent), Li 2 O (1 to 25 percent), ZnO (0 to 20 percent), La 2 O 3  (4 to 30 percent), Gd 2 O 3  (1 to 25 percent), Y 2 O 3  (0 to 20 percent), ZrO 2  (0 to 5 percent), MgO (0 to 25 percent), CaO (0 to 15 percent), and SrO (0 to 10 percent), with the combined quantity of the above components being 97 percent or greater. The molar ratio of {ZnO/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 )} is 0.8 or less and the molar ratio of {(CaO+SrO+BaO)/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 )} is 0.8 or less. Ta 2 O 5  may be incorporated as an optional component, with the molar ratio {(ZrO 2 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 )} being 0.4 or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of application Ser. No. 13/230,914 filed Sep. 13,2011, which is a Continuation of application Ser. No. 11/727,736 filedMar. 28, 2007, now U.S. Pat. No. 8,039,408, issued on Oct. 18, 2011,claiming priority based on Japanese Patent Application No. 2006-089116,filed on Mar. 28, 2006, the contents of all of which are incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical glass having the opticalconstants of a refractive index n_(d) of 1.70 or greater and an Abbénumber ν_(d) of 50 or greater, a preform for precision press moldingcomprised of this glass, an optical element comprised of this glass, andmethods of manufacturing the same.

2. Discussion of the Background

With the advent of digital cameras and cellular telephones equipped withcameras, the degree of integration and the high degree of functionalityof devices employing optical systems have advanced rapidly. At the sametime, the demand for high precision, lightweight, compact opticalsystems is increasing.

In recent years, to meet the above need, optical designs employingaspherical lenses have been increasingly coming into the mainstream.Thus, to stably supply large numbers of aspherical lenses employinghighly functional glass at low cost, precision press molding (also knownas a mold pressing technique), in which an optical functional surface isdirectly formed by press molding without grinding or polishing steps,has attracted attention. The demand for optical glass having lowtemperature softening properties suited to precision press molding isincreasing each year. Among such optical glasses, there are highrefractive index, low dispersion glasses. An example of such a glass isdescribed in Japanese Unexamined Patent Publication (KOKAI) No.2002-249337 or English language family member U.S. Patent ApplicationNo. 2003-125186 AA, which are expressly incorporated herein by referencein their entirety.

To take full advantage of the above precision press molding technique, aglass material known as a preform is desirably directly formed fromglass melt. This method is known as the preform hot molding method. Aglass melt is caused to flow out, glass melt gobs of a weightcorresponding to single preforms are separated one after another, andthe glass melt gobs obtained are cooled to form preforms having smoothsurfaces. Accordingly, in contrast to the method of forming large glassblocks from glass melt and cutting, grinding, and polishing the glassblocks, this method affords excellent characteristics in the form of ahigher glass use rate, the fact that no glass shavings are producedduring processing, and zero processing time and cost.

However, in the hot molding method, a glass melt gob of a weightcorresponding to a single preform is accurately separated and a preformmolded in such a manner that devitrification and defects such as striaedo not result. Accordingly, a glass having good glass stability in thehigh temperature range of hot molding is required.

When the refractive index n_(d) is raised while maintaining an Abbénumber ν_(d) of a prescribed level or above, the tendency of the glassto crystallize intensifies, thereby compromising vitrification. Since alow temperature softening property is further imparted to the glassemployed in precision press molding, there tends to be a drop in glassstability. Accordingly, it is difficult to realize glass stability at alevel permitting the hot molding of preforms while maintaining an Abbénumber ν_(d) of 50 or greater, preferably 52 or greater, a refractiveindex n_(d) of 1.70 or greater, and a low temperature softening propertysuited to precision press molding.

The present invention provides an optical glass exhibiting good glassstability while having a refractive index nd of 1.70 or greater, an Abbénumber ν_(d) of 50 or greater, and a low temperature softening property.The present invention further relates to a preform for precision pressmolding comprised of the above glass, an optical element comprised ofthe above glass. The present invention further provides a method formanufacturing the preform for precision press molding comprised of theabove glass and a method for manufacturing the optical element comprisedof the above glass.

SUMMARY OF THE INVENTION

The present invention relates to an optical glass having a refractiveindex n_(d) of 1.70 or greater and an Abbé number of 50 or greater, and,given as mole percentages, comprising:

B₂O₃ 20 to 80 percent, SiO₂  0 to 30 percent, Li₂O  1 to 25 percent; ZnO 0 to 20 percent, La₂O₃  4 to 30 percent, Gd₂O₃  1 to 25 percent, Y₂O₃ 0 to 20 percent, ZrO₂  0 to 5 percent, MgO  0 to 25 percent, CaO  0 to15 percent, SrO  0 to 10 percent,with the combined quantity of the above components being 97 percent orgreater; the molar ratio of {ZnO/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.8 or lessand the molar ratio of {(CaO+SrO+Bao)/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.8 orless; Ta₂O₅ may be incorporated as an optional component, with the molarratio {(ZrO₂+Ta₂O₅)/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.4 or less.

The present invention provides an optical glass exhibiting good glassstability while having a refractive index n_(d) of 1.70 or greater, anAbbé number ν_(d) of 50 or greater, and a low temperature softeningproperty. The present invention further relates to a preform forprecision press molding comprised of the above glass, an optical elementcomprised of the above glass. The present invention further provides amethod for manufacturing the preform for precision press moldingcomprised of the above glass and a method for manufacturing the opticalelement comprised of the above glass.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in the following text by theexemplary, non-limiting embodiments shown in the figures, wherein:

FIG. 1 is a descriptive sectional drawing of a precision pressing mold.

-   1: Upper mold-   2: Lower mold-   3: Sleeve mold-   4: Preform for precision press molding-   9: Support rod-   10: Support member-   11: Quartz tube-   13: Pressing rod-   14: Thermocouple

DESCRIPTIONS OF THE EMBODIMENTS

The following preferred specific embodiments are, therefore, to beconstrued as merely illustrative, and not limitative of the remainder ofthe disclosure in any way whatsoever. In this regard, no attempt is madeto show structural details of the present invention in more detail thanis necessary for the fundamental understanding of the present invention,the description taken with the drawings making apparent to those skilledin the art how the several forms of the present invention may beembodied in practice.

The present invention is described in detail below.

[Optical Glass]

The optical glass of the present invention has a refractive index n_(d)of 1.70 or greater and an Abbé number of 50 or greater. Given as molepercentages, it comprises:

B₂O₃ 20 to 80 percent, SiO₂  0 to 30 percent, Li₂O  1 to 25 percent; ZnO 0 to 20 percent, La₂O₃  4 to 30 percent, Gd₂O₃  1 to 25 percent, Y₂O₃ 0 to 20 percent, ZrO₂  0 to 5 percent, MgO  0 to 25 percent, CaO  0 to15 percent, SrO  0 to 10 percent,with the combined quantity of the above components being 97 percent orgreater. The molar ratio of {ZnO/(La₂O₃+Gd₂O₃+Y₂O₃)} is 0.8 or less andthe molar ratio of {(CaO+SrO+BaO)/(La₂O₃+Gd₂O₃+Y₂O₃)} is 0.8 or less.Ta₂O₅ may be incorporated as an optional component, with the molar ratio{(ZrO₂+Ta₂O₅)/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.4 or less.

Unless specifically stated otherwise, the various contents and combinedquantities given below are mole percentages, and the content ratios,combined content ratios, and content-to-combined content ratios aremolar ratios.

In the present invention, the molar ratio of{(CaO+SrO+BaO)/(La₂O₃+Gd₂O₃+Y₂O₃)} refers to the ratio of the combinedcontent of CaO, SrO, and BaO to the combined content of La₂O₃, Gd₂O₃ andY₂O₃. The molar ratio of {ZnO/(La₂O₃+Gd₂O₃+Y₂O₃)} refers to the ratio ofZnO to the combined content of La₂O₃, Gd₂O₃, and Y₂O₃. And the molarratio of {(ZrO₂+Ta₂O₅)/(La₂O₃+Gd₂O₃+Y₂O₃)} refers to the ratio of thecombined content of ZrO₂ and Ta₂O₅ to the combined content of La₂O₃,Gd₂O₃, and Y₂O₃.

B₂O₃, a glass network-forming component, is an essential componentfunctioning to impart a low dispersion property and lower the glasstransition temperature. At a content of less than 20 percent, thestability of the glass decreases, the liquidus temperature rises, and itbecomes difficult to mold preforms. Conversely, the refractive indexdecreases when an excessively large quantity is incorporated.Accordingly, the content of B₂O₃ in the present invention is set to 20to 80 percent, with a range of 25 to 75 percent being desirable and arange of 25 to 72 percent being preferable.

SiO₂ is an optional component the incorporation of which in suitablequantity enhances the stability of the glass and functions to impart aviscosity suitable for molding when molding preforms out of glass melt.However, the refractive index drops and the meltability of the glassdecreases when incorporated in excessively large quantity. Accordingly,the content is set to 0 to 30 percent, desirably 1 to 27 percent,preferably 2 to 25 percent.

LiO₂ is an essential component having the functions of increasing therefractive index and lowering the glass transition temperature moreefficiently than other alkali metal oxides. It also improves themeltability of the glass. When incorporated in excessively smallquantity, these effects are compromised; when incorporated inexcessively large quantity, the resistance to devitrification of theglass decreases, it becomes difficult to directly mold high-qualitypreforms from a glass melt flow, and weatherability deteriorates.Accordingly, the content is set to 1 to 25 percent, desirably 2 to 20percent, and preferably, 3 to 18 percent.

From the perspectives of the glass transition temperature and thestability of the glass, it is desirable to optimize the ratio of Li₂O tothe network-forming components; thus, the molar ratio ofLi₂O/(B₂O₃+SiO₂) is desirably set to the range of 0.02 to 0.25,preferably to the range of 0.04 to 0.20, and more preferably, to therange of 0.06 to 0.18.

ZnO is a component functioning to lower the melting temperature,liquidus temperature, and glass transition temperature; enhance thechemical durability and weatherability of the glass; and raise therefractive index. However, when incorporated in excessively largequantity, it becomes difficult to maintain an Abbé number ν_(d) of 50 orgreater. Thus, the content is set to 0 to 20 percent, desirably 0 to 16percent, and more preferably, 1 to 14 percent.

To achieve the desired low temperature softening property in the opticalglass of the present invention, it is desirable for the combined contentof Li₂O and ZnO (Li₂O+ZnO) to be 2 percent or greater. However, whenthis combined content is excessively high, the resistance of the glassto devitrification decreases and dispersion increases. Thus, Li₂O+ZnO isdesirably set to 2 to 30 percent, preferably 3 to 25 percent, and morepreferably, 4 to 23 percent.

To maintain an Abbé number ν_(d) of 50 or greater while imparting thedesired low temperature softening property, the molar ratio of ZnO/Li₂Ois desirably set to 6 or less, preferably 0 to 5, and more preferably0.2 to 4.

La₂O₃ is an essential component that functions to enhance durability andweatherability while maintaining a low dispersion property and raisingthe refractive index. However, when introduced in excessively highquantity, it lowers the stability of the glass and raises the glasstransition temperature. Thus, the content is set to 4 to 30 percent,desirably 4 to 25 percent, preferably 5 to 22 percent, and morepreferably, 6 to 20 percent.

Gd₂O₃ is an essential component functioning in the same manner as La₂O₃.However, when incorporated in excessively high quantity, it lowers thestability of the glass and raises the glass transition temperature.Thus, the content is set to 1 to 25 percent, desirably 1 to 20 percent,preferably 1 to 18 percent, more preferably 2 to 18 percent, and stillmore preferably, 3 to 16 percent.

From the perspective of increasing the stability of the glass, the ratioof La₂O₃ to Gd₂O₃ is desirably adjusted. As a molar ratio, La₂O₃/Gd₂O₃desirably falls within a range of 0.5 to 5.0, preferably a range of 0.8to 4.8, more preferably range of 0.8 to 4.6, still more preferably arange of 0.9 to 4.4, and still more preferably, a range of 1.0 to 4.2.

Y₂O₃ is an optional component functioning in the same manner as La₂O₃and Gd₂O₃. The incorporation of a small quantity advantageously raisesthe heat resistance of the glass and lowers the liquidus temperature.However, the incorporation of an excessively large quantity lowers thestability of the glass and raises the glass transition temperature.Thus, the content is set to 0 to 20 percent, desirably 0.1 to 20percent, preferably 0.2 to 20 percent, more preferably 0.3 to 20percent, still more preferably 0.3 to 15 percent, and still morepreferably, 0.3 to 10 percent. To achieve a particularly high refractiveindex of n_(d)≧1.7 and low dispersion property of ν_(d)≧52, the contentis desirably 0.5 to 20 percent, preferably 0.5 to 15 percent, and morepreferably, 1 to 12 percent. To achieve good thermal stability in theglass while maintaining a high refractive index and a low dispersionproperty, Y₂O₃ is desirably incorporated as an essential component.

The optical glass of the present invention desirably comprises glasscomponents in the form of La₂O₃, Gd₂O₃, and Y₂O₃ in combination.Incorporating three or more of such rare earth oxide componentsheightens the stability of the glass more than when two or fewer rareearth oxide components are employed.

ZrO₂ is an optional component that can be incorporated to enhance theweatherability of the glass and adjust optical constants. Theincorporation of a small quantity functions to enhance the stability ofthe glass. However, the incorporation of an excessively large quantitydecreases the stability of the glass and increases dispersion. Thus, thecontent is desirably 0 to 5 percent, preferably 0 to 4.5 percent, andmore preferably, 0 to 4 percent. To achieve a particularly highrefractive index of n_(d)≧1.7 and low dispersion property of ν_(d)≧52,the content is desirably 0 to 3 percent, preferably 0 to 2 percent, morepreferably 0 to 1.5 percent, and still more preferably, 0 to 0.5percent.

When MgO is incorporated instead of ZnO or Li₂O, low glass dispersionand high chemical durability of the glass can be achieved. However,incorporation in excessively high quantity lowers the refractive indexand raises the glass transition temperature. Thus, the content is set to0 to 25 percent, desirably 0 to 20 percent, and preferably, 0 to 15percent.

CaO lowers the glass transition temperature and adjusts opticalcharacteristics. However, incorporation in excessively large quantitylowers the stability of the glass and raises the liquidus temperature.Thus, the content is set to 0 to 15 percent, desirably 0.2 to 14percent, and preferably, 0.5 to 12 percent.

SrO enhances chemical durability and adjusts optical characteristics.However, incorporation in excessively high quantity lowers the stabilityof the glass and raises the liquidus temperature. Thus, the content isset to 0 to 10 percent, desirably 0 to 5 percent, and preferably, 0 to 3percent.

From the perspectives of raising the refractive index and lowering thedispersion of the optical glass of the present invention, the molarratio {ZnO/(La₂O₃+Gd₂O₃+Y₂O₃)} is set to 0.8 or less. When this molarratio exceeds 0.8, it becomes difficult to achieve desired opticalconstants. This molar ratio is desirably 0.75 or less, preferably 0.7 orless, more preferably 0.65 or less, and still more preferably, 0.6 orless.

The incorporation of components having an ion radius that is smallerthan that of components having a large ion radius as alkaline earthcomponents is desirable from the perspective of simultaneously achievingboth an n_(d)≧1.7 and glass stability. Thus, in the optical glass of thepresent invention, the molar ratio {(CaO+SrO+BaO)/(La₂O₃+Gd₂O₃+Y₂O₃)} is0.8 or less. When this molar ratio exceeds 0.8, it becomes difficult tosimultaneously achieve both a high refractive index and glass stability.This molar ratio is desirably 0.6 or less, preferably 0.5 or less, andmore preferably, 0.4 or less.

In the optical glass of the present invention, the combined content ofthe above components is set to 97 percent or greater. When componentsother than the above-described components are incorporated in largequantity into the optical glass of the present invention, problems suchas loss of the low dispersion property, loss of the high refractiveindex property, and loss of glass stability are encountered. Thus, thetotal content of the above-described components is desirably high. Theabove combined content is desirably 98 percent or greater, preferably 99percent or greater, and more preferably, 100 percent.

Examples of other components are Ta₂O₅, F, Al₂O₃, Yb₂O₃, Sc₂O₃, andLu₂O₃.

Ta₂O₅ functions to raise the refractive index, but also increasesdispersion. Thus, the quantity incorporated is controlled. In theoptical glass of the present invention, the components that raise therefractive index—La₂O₃, Gd₂O₃, Y₂O₃, ZrO₂, and Ta₂O₅—are divided into agroup that maintains a low dispersion property (La₂O₃, Gd₂O₃, and Y₂O₃)and a group that increases dispersion (ZrO₂ and Ta₂O₅). The quantity ofoptional component Ta₂O₅ incorporated is limited by optimizing the ratioof the total quantity of each group. That is, in the optical glass ofthe present invention, the molar ratio of{(ZrO₂+Ta₂O₅)/(La₂O₃+Gd₂O₃+Y₂O₃)} is set to 0.4 or less. When this molarratio exceeds 0.4, it becomes difficult to maintain a low dispersionproperty. This molar ratio is desirably 0.3 or lower, preferably 0.27 orlower, more preferably 0.255 or lower, more preferably 0.225 or lower,still more preferably 0.2 or lower, still more preferably 0.18 or lower,still more preferably 0.1 or lower, still more preferably 0.05 or lower,and most preferably, zero.

For the above-stated reasons, the Ta₂O₅ content is desirably kept towithin a range of 0 to 3 percent, preferably 0 to 2 percent, morepreferably 0 to 1 percent, still more preferably 0 to 0.5 percent, stillmore preferably 0 to 0.2 percent, still more preferably 0 to 0.1percent, and most preferably, zero.

To improve the thermal stability of the glass while maintaining a highrefractive index and a low dispersion property, it is desirable to:incorporate Y₂O₃ as set forth above as an essential component; keep{(ZrO₂+Ta₂O₅)/La₂O₃+Gd₂O₃+Y₂O₃)} low; reduce the content of Ta₂O₅; ornot incorporate Ta₂O₅.

In the composition of a B₂O₃—La₂O₃ system, in terms of opticalcharacteristics, fluorine increases the range over which vitrificationis possible and lowers the glass transition temperature. However, whenpresent with B₂O₃, fluorine exhibits marked volatization at hightemperatures, volatizing during melting of the glass and during molding.It thus renders difficult the mass production of glass having a constantrefractive index. Further, volatile matter from the glass adheres to thepressing mold during precision press molding. When such a mold isrepeatedly employed, there is a problem in that the surface precision ofthe lenses decreases. Accordingly, the content of fluorine is desirablykept to 10 percent or less, preferably to 5 percent or less. In methodsof directly molding preforms from glass melt, striae due to volatizationoccur, making it difficult to obtain an optically homogeneous preform.Thus, the content of fluorine is desirably kept to 3 percent or less,with no incorporation being preferred.

Al₂O₃ serves to enhance chemical durability. However, when incorporatedin excessively high quantity, it lowers the refractive index and raisesthe glass transition temperature. Accordingly, the content is desirably0 to 10 percent, preferably 0 to 8 percent, and more preferably, 0 to 5percent.

Sc₂O₃ functions in the same manner as La₂O₃, Gd₂O₃, and Y₂O₃. Theincorporation of a small quantity advantageously increases the thermalstability of the glass and lowers the liquidus temperature. However,these advantages are lost, the stability of the glass decreases, and therefractive index drops with the incorporation of an excessive quantity.Sc₂O₃ is an expensive component and is desirably not employed from theperspective of cost, depending on the industrial application. On thisbasis, the quantity is desirably 0 to 20 percent, preferably 0 to 18percent, more preferably 0 to 13 percent, still more preferably 0.1 to10 percent, and most preferably, 0.1 to 5 percent.

Yb₂O₃ and Lu₂O₃ can each be incorporated. However, they reduce thethermal stability of the glass and greatly increase the liquidustemperature. Thus, the content of Yb₂O₃ is kept to within the range of 0to 5 percent, preferably 0 to 2 percent, more preferably 0 to 1 percent,and still more preferably, 0 to 0.5 percent. The content of Lu₂O₃, aswell, is kept to within the range of 0 to 5 percent, preferably 0 to 2percent, more preferably 0 to 1 percent, and still more preferably, 0 to0.5 percent. Both Yb₂O₃ and Lu₂O₃ are expensive components and need notnecessarily be employed in the optical glass of the present invention.Thus, from the perspective of cost, both Yb₂O₃ and Lu₂O₃ are desirablynot incorporated.

GeO₂ may be incorporated in a range of 0 to 10 percent, for example.Since it is an expensive component, the quantity incorporated isdesirably kept to 5 percent or less, with no incorporation beingpreferred.

BaO greatly reduces the stability of the glass when incorporated insmall quantity. Thus, the content is desirably limited to 2 percent orless, with no incorporation being preferred.

Nb₂O₅ and TiO₂ both have the effect of strongly increasing dispersionand greatly raising the Abbé number nu_(d) when incorporated in onlysmall quantities. Thus, to maintain an Abbé number ν_(d) of 50 orgreater, the quantity of Nb₂O₃ is desirably kept to 2 percent or less,preferably 1 percent or less, with no incorporation at all beingpreferred overall. To maintain an Abbé number ν_(d) of 50 or greater,the quantity of TiO₂ is desirably kept to 2 percent or less, preferably1 percent or less, with no incorporation at all being preferred overall.

WO₃ and Bi₂O₃ both have the same effects as Nb₂O₅ and TiO₂. Thus, thequantity of each employed is desirably kept to 2 percent or less,preferably 1 percent or less, with no incorporation at all beingpreferred overall.

Not only do Nb₂O₅, TiO₂, WO₃, and Bi₂O₃ all increase dispersion, butthey also increase coloration of the glass. Since the optical glass ofthe present invention, as is the case for optical glass in general, hasgood light transparency, it is desirable not to incorporate Nb₂O₅, TiO₂,WO₃, and Bi₂O₃ to avoid using such substances.

When negative effects on the environment are considered, theincorporation of Pb, Cr, Cd, As, Th, Te, and U is avoided. Pb has beenconventionally employed as a main component to raise the refractiveindex of optical glass. However, in addition to the above problem, leadis readily reduced during precision press molding in a nonoxidizing gasatmosphere, with precipitating metallic lead adhering to the moldingsurfaces of the pressing mold and causing problems such as reducing thesurface precision of the press molded product. As₂O₃ has also beenconventionally added as a clarifying agent, but in addition to theabove-described problems, causes problems by oxidizing the moldingsurface of the pressing mold and shortening the service life of themold. Thus, it is not incorporated.

Substances that color the glass, such as Fe, Cu, and Co, are desirablynot incorporated unless with the object of imparting desired spectralcharacteristics to the glass.

Sb₂O₃ is an optional additive employed as a clarifying agent. Theaddition of a small quantity diminishes absorption by reducingimpurities such as Fe and suppresses coloration of the glass. However,the addition of an excessively large quantity both results in the lossof this effect and oxidizes the molding surface of the pressing moldduring precision press molding, negatively affecting the service life ofthe pressing mold. Such addition is thus undesirable for precision pressmolding. Accordingly, as an externally calculated ratio, the quantityadded is desirably 0 to 0.5 weight percent, preferably 0 to 0.2 weightpercent.

The optical glass of the present invention has a refractive index n_(d)of 1.70 or greater and an Abbé number ν_(d) of 50 or more. Glasseshaving optical constants falling within these ranges that are employedas materials for optical elements desirably have relatively highrefractive indexes and relatively low dispersion (high Abbé numbers).Additionally, when the refractive index is raised and the dispersion islowered while maintaining a low glass transition temperature, thestability of the glass decreases markedly. Thus, it has conventionallybeen difficult to raise the refractive index and lower dispersion evenfurther in this range. By contrast, since the optical glass of thepresent invention is optimized as a glass for precision press molding,it is possible to achieve a refractive index n_(d) and Abbé number ν_(d)satisfying equation (1) below:

n _(d)≧2.235−0.1×ν_(d)  (1)

When directly molding preforms for precision press molding from glassmelt gobs, an excessively high refractive index and an excessively lowdispersion are undesirable in order to maintain good glass stability.The optical properties are desirably set to ranges satisfying equation(2) below, and preferably set to ranges satisfying equation (3) below.However, even in these cases, the setting of optical properties toranges satisfying equation (1) is preferred.

n _(d)≧2.285−0.1×ν_(d)  (2)

n _(d)≧2.275−0.1×ν_(d)  (3)

The optical glass of the present invention is a low dispersion glasswith an Abbé number ν_(d) of 50 or greater. From the perspective of alow glass dispersion property, the Abbé number ν_(d) is desirably 51 orgreater, preferably 52 or greater, more preferably 52.5 or greater,still more preferably 53 or greater, and most preferably, 54 or greater.

The optical glass of the present invention can achieve a low glasstransition temperature suited to precision press molding. The glasstransition temperature desirably falls within a range of 635° C. andbelow, preferably 620° C. and below. However, when the glass transitiontemperature is excessively low, higher refractive indexes and lowerdispersion become harder to achieve, and/or the stability and chemicaldurability of the glass tends to decrease. Thus, the glass transitiontemperature is desirably 535° C. or above, preferably 555° C. or above,and more preferably, 565° C. or above.

The optical glass of the present invention has good glass stability. Forexample, taking the stability in the high temperature range required formolding glass from glass melt as a yardstick, glass having a liquidustemperature of 1090° C. or lower can be achieved. In this manner, in theoptical glass of the present invention, it is possible to keep theliquidus temperature below a prescribed temperature in a high refractiveindex, low dispersion glass. Thus, it is possible to directly moldpreforms for precision press molding from glass melt. The liquidustemperature desirably falls within a range of 1060° C. and below,preferably 1050° C. and below, and more preferably, 1040° C. and below.

As set forth above, the optical glass of the present invention exhibitsgood light transparency. Quantitatively, a low degree of coloration isrealized in the form of a λ₈₀ (nm) that is, for example, 410 nm orlower, desirably 400 nm or lower, preferably 390 nm or lower, morepreferably 380 nm or lower, still more preferably 370 or lower, stillmore preferably 360 or lower, and still more preferably, 350 or lower.λ₈₀ (nm) is calculated as follows. A glass sample having opticallypolished parallel flat surfaces that are 10.0±0.1 mm in thickness isemployed. Intense light Iin is directed perpendicularly onto one of theflat surfaces, the intensity of the light Iout exiting from the otherflat surface is measured, and the external transmittance (Iout/Iin) iscalculated. The external transmittance is measured over the wavelengthrange of 280 to 700 nm. λ₈₀ (nm) is the wavelength at which the externaltransmittance reaches 80 percent. Common optical glass to which coloringagents are not added, such as the optical glass of the presentinvention, exhibit little absorption on the long wavelength side at theabsorption end from the ultraviolet to the visible region. Thus, in aglass sample having optically polished parallel flat surfaces with athickness of 10.0±0.1 mm, an internal transmittance exceeding 80 percentis obtained over the wavelength region from λ₈₀ (nm) to 1550 nm. In aglass sample having optically polished parallel flat surfaces with athickness of 10.0±0.1 mm, an internal transmittance exceeding 90 percentis obtained over the long wavelength region from λ₈₀+20 (nm) to 1550 nm.The λ₇₀ (nm) and λ₅ (nm) in Table 1 below are wavelengths with externaltransmittances of 70 percent and 5 percent, respectively, that werecalculated based on the λ₈₀ method.

The optical glass of the present invention is suitable as a material forlenses used to configure image pickup optical systems, as well as beingsuitable as a material for lenses used to configure optical systemsemployed in the recording and reproduction of optical disks such as DVDsand CDs. As an example, the optical glass of the present invention issuitable as a material for optical elements employed to record andreproduce data with blue-violet light (for example, a semiconductorlaser beam with a wavelength of 405 nm) to achieve excellent opticaltransparency. Specifically, for example, it is suited to objectivelenses for DVDs with a high recording density of 23 GB. Asphericallenses with numerical apertures of 0.85 are currently the mainstream insuch objective lenses. Such lenses have a high ratio of center thicknessto effective diameter; however, the optical glass of the presentinvention permits a reduction in this ratio by having a high refractiveindex while maintaining a low dispersion property. Since the thicknessof lenses passing blue-violet light can be diminished, the loss ofblue-violet light can be reduced while maintaining good lighttransparency in the glass. Further, a reduction in the ratio of thecenter thickness to the effective diameter is desirable for precisionpress molding. That is, the volume of the preform employed in precisionpress molding is determined by the volume of the lens. Since the aboveobjective lenses are small, the preforms employed in molding may bespherical or ellipsoids of revolution. When employing a sphericalpreform and the curvature of the convex surface of the lens is high (theradius of curvature is small), atmospheric gas becomes trapped betweenthe pressing mold and the glass, and a problem called gas trapping,where the gas does not diffuse, tends to occur in such spots. Reducingthe ratio of the center thickness to the effective diameter is linked toincreasing the curvature of the convex surface of the lens, and is thusdesirable when fabricating lenses of high surface precision by precisionpress molding.

The method for manufacturing the optical glass of the present inventionwill be described next. The optical glass of the present invention canbe manufactured by heating and melting glass starting materials. Theglass starting materials may be in the form of suitable carbonates,nitrates, oxides, and the like. These starting materials are weighed outin prescribed proportions and mixed to obtain a blended startingmaterial. This blended starting material is then charged to a meltingfurnace that has been heated to 1,200 to 1,300° C., for example, andmelted, clarified, stirred, and homogenized to obtain a homogeneousglass melt free of bubbles and unmelted matter. The glass melt is moldedand gradually cooled to obtain the optical glass of the presentinvention.

[Preform for Precision Press Molding and Method for ManufacturingPreforms for Precision Press Molding]

The preform for precision press molding and method for manufacturingpreforms for precision press molding of the present invention will bedescribed next. The preform for precision press molding can be referredto as the precision press molding preform or simply as the preform.

The preform is a molded glass member equal in weight to a precisionpress molded article. The preform is molded into a suitable shapecorresponding to the shape of a precision press molded article. Examplesof such shapes are spheroids and ellipsoids of revolution. The preformis heated to a temperature at which it exhibits a viscosity permittingprecision press molding and is then precision press molded.

The preform for precision press molding of the present invention iscomprised of the above-described optical glass of the present invention.As needed, the surface of the preform of the present invention may beprovided with a thin film such as a mold release agent. The preformpermits precision press molding of optical elements having desiredoptical constants. Since the glass is stable in the high temperaturerange and the viscosity of the glass melt when flowing out can beincreased, in methods of molding preforms in a step of cooling a glassgob obtained by separation from a glass melt caused to flow out of apipe, an advantage is afforded in that high-quality preforms can bemanufactured with high productivity.

The method for manufacturing preforms for precision press molding of thepresent invention, one way of manufacturing the preform of the presentinvention, is a method for manufacturing preforms for precision pressmolding that are comprised of the optical glass of the present inventionby separating glass melt gobs from an outflowing glass melt and moldingthe glass melt gobs into preforms for precision press molding in acooling step. Specifically, it is a manufacturing method for moldingpreforms of prescribed weight by the steps of separating glass melt gobsof prescribed weight from a glass melt flow exiting a pipe or the likeand cooling the glass gobs. This method is advantageous in that nomechanical processing such as cutting, grinding, or polishing isrequired. Mechanically processed preforms are annealed prior tomechanical processing to reduce distortion of the glass to a degree atwhich the glass will not be damaged. However, annealing to preventdamage is not required by the above method. Further, preforms withsmooth surfaces can also be molded. In this method, from the perspectiveof imparting smooth, clean surfaces, the preforms are desirably floatedby applying wind pressure. Further, the outer surfaces of the preformsare desirably comprised of free surfaces. Still further, preforms freeof cutting traces known as shear marks are desirable. Shear marks areproduced when the outflowing glass melt is cut with a blade. When shearmarks remain at the stage of molding precision press molded articles,they end up becoming defects. Thus, shear marks are desirably removed bythe preform stage. Methods of separating glass melt without producingshear marks include: the method of dripping glass melt from an outflowpipe, and the method of supporting the front end portion of a glass meltflow as it flows out of an outflow pipe and removing this support at atiming permitting the separation of a glass melt gob of prescribedweight (referred to as the “drop cut method” hereinafter). In the dropcut method, the glass can be separated at a constriction formed betweenthe front end of the glass melt flow and the end of the outflow pipe toobtain a glass melt gob of prescribed weight. The glass melt gobobtained is then molded while still soft into a suitable shape for feedinto a pressing mold.

The preform of the present invention can be manufactured by formingmolded glass members out of glass melt, and then cutting or breaking;grinding; and polishing these molded members. In this method, a preformof desired weight is fabricated by molding a molded glass membercomprised of the above-described optical glass by causing a glass meltto flow into a casting mold, and then mechanically processing the moldedglass member. Prior to conducting mechanical processing, relativelythorough processing to relieve distortion is desirably conducted byannealing the glass to prevent damaging the glass.

[Optical Element and Method for Manufacturing Optical Elements]

The optical element of the present invention is comprised of theabove-described optical glass of the present invention. The opticalelement of the present invention is characterized by a high refractiveindex and low dispersion in the same manner as the optical glass of thepresent invention constituting the optical element.

Examples of the optical element of the present invention are variouslenses such as spherical lenses, aspherical lenses, and microlenses;diffraction gratings; lenses with diffraction gratings; lens arrays, andprisms. From the viewpoint of applications, examples are: the lensesconstituting image pickup optical systems such as digital still cameras,digital video cameras, single-lens reflex cameras, cameras mounted onportable telephones, and vehicle-mounted cameras; and lensesconstituting optical systems for reading and writing data to and fromoptical disks such as DVDs and CDs (for example, the above-describedobjective lenses).

The above-described optical element is desirably obtained by heatsoftening and precision press molding the preform of the presentinvention.

As needed, an optical thin film such as an antireflective film, fullyreflective film, partially reflective film, or a film having spectralproperties may be provided on the optical element.

The method for manufacturing optical elements of the present inventionwill be described next.

In the method for manufacturing optical elements of the presentinvention, the preform of the present invention or a preform forprecision press molding manufactured by the method for manufacturingpreforms of the present invention is heated and precision press moldedto produce an optical element.

The precision press molding method, also known as the mold opticsmolding method, is already well known in the field of art to which thepresent invention pertains.

Any surface of an optical element that transmits, refracts, diffracts,or reflects rays of light is called an optically functional surface. Inthe example of a lens, lens surfaces such as the aspherical surface ofan aspherical lens and the spherical surface of a spherical lens bothcorrespond to optically functional surfaces. In precision press molding,the molding surface of the pressing mold is precisely transferred to theglass to form an optically functional surface by press molding. That is,to finish an optically functional surface, no mechanical processing suchas grinding or polishing is required.

Accordingly, the method for manufacturing optical elements of thepresent invention is suited to the manufacturing of optical elementssuch as lenses, lens arrays, diffraction gratings, and prisms, and isoptimal for the manufacturing of aspherical lenses with highproductivity.

The method for manufacturing optical elements of the present inventionpermits the manufacturing of optical elements having the above-describedoptical properties and permits adjustment of the glass composition asset forth above to impart low temperature processing properties to thepreform, thereby permitting press molding of the glass at relatively lowtemperature. Thus, the load on the molding surface of the pressing moldis reduced and the service lifetime of the pressing mold (or moldrelease film when a mold release film is provided on the moldingsurface) is extended. Adjustment of the glass composition increases thestability of the glass constituting the preform, making it possible toeffectively prevent devitrification of the glass during the reheatingand pressing steps. Further, the entire series of steps from melting theglass to obtaining the final product can be conducted with highproductivity.

A known pressing mold may be employed for precision press molding, suchas a mold made of a material such as silicon carbide, an ultrahardmaterial, or stainless steel having molding surfaces that have beencoated with a mold release film. The mold release film employed may be acarbon-containing film, a noble metal alloy film, or the like. Thepressing mold is equipped with upper and lower molds, and as necessary,a drum mold. Of these, to effectively reduce or prevent damage to glassmolded articles during press molding, the use of a pressing moldcomprised of silicon carbide or an ultrahard alloy (particularly onemade of an ultrahard alloy not containing binder, such as a pressingmold made of WC) is desirable. The providing of a mold release film inthe form of a carbon-containing film on the molding surface of the moldis also desirable.

Precision press molding is desirably conducted using a non-oxidizing gasatmosphere during molding to keep the molding surfaces of the pressingmold in good condition. Examples of preferred non-oxidizing gases arenitrogen and mixtures of nitrogen and hydrogen. Particularly whenemploying a pressing mold with molding surfaces equipped with a moldrelease film in the form of a carbon-containing film and when employinga pressing mold comprised of silicon carbide, it is necessary to conductprecision press molding in such a non-oxidizing atmosphere.

Precision press molding methods particularly suited to the method formanufacturing an optical element of the present invention will bedescribed next.

(Precision Press Molding Method 1)

In this method, a preform is introduced to the pressing mold, thepressing mold and preform are both heated, and precision press moldingis conducted (“Precision Press Molding Method 1” hereinafter).

In Precision Press Molding Method 1, the pressing mold and preform areboth desirably heated to a temperature at which the glass constitutingthe preform exhibits a viscosity of 10⁶ to 10¹² dPa·s to conductprecision press molding.

The precision press molded article is desirably removed from thepressing mold after being cooled to a temperature at which the glassexhibits a viscosity of 10¹² dPa·s or more, preferably 10¹⁴ dPa·s ormore, and more preferably, 10¹⁶ dPa·s or more.

Under these conditions, the shape of the molding surfaces of thepressing mold can be precisely transferred to the glass and theprecision press molded article can be removed without deformation.

(Precision Press Molding Method 2)

This method is characterized in that a preform that has been heated toone temperature is introduced into a pressing mold that has beenpreheated to another temperature and the preform is precision pressmolded (“Precision Press Molding Method 2” hereinafter). This methodallows the preform to be preheated prior to being introduced into thepressing mold, thereby shortening the cycle time and permitting themanufacturing of optical elements that have good surface precision andare free of surface defects.

The temperature to which the pressing mold is preheated is desirablylower than that to which the preform is preheated. Such preheating keepsdown the temperature to which the pressing mold is heated, therebyreducing wear and tear on the pressing mold.

In Precision Press Molding Method 2, the preform is desirably preheatedto a temperature at which the glass constituting the preform exhibits aviscosity of 10⁹ dPa·s or less, preferably 10⁹ dPa·s.

Further, the preform is desirably preheated while being floated;preheating is preferably conducted to a temperature at which the glassconstituting the preform exhibits a viscosity of 10^(5.5) to 10⁹ dPa·s,more preferably greater than or equal to 10^(5.5) but less than 10⁹dPa·s.

Cooling of the glass is desirably begun simultaneously with the startof, or during, pressing.

The temperature of the pressing mold is desirably adjusted to below thetemperature to which the preform is preheated. It suffices to use atemperature at which the glass exhibits a viscosity of 10⁹ to 10¹² dPa·sas yardstick.

In this method, the press molded article is desirably removed from themold after being cooled to a temperature at which the glass exhibits aviscosity of 10¹² dPa·s or more following press molding.

The optical element that has been precision press molded is removed fromthe pressing mold and gradually cooled as necessary. When the moldedarticle is an optical element such as a lens, an optical film may becoated on the surface thereof as needed.

EXAMPLES

The present invention will be further described below through examples.However, the present invention is not limited to the forms shown in theexamples.

Manufacturing Optical Glass

Table 1 shows the compositions of the glasses of examples 1 to 30. Foreach of these glasses, starting materials of the various components inthe form of corresponding oxides, hydroxides, carbonates, and nitrateswere weighed out to yield the compositions shown in Table 1 followingvitrification, thoroughly mixed, charged to a platinum crucible, meltedat a temperature range of from 1,200 to 1,300° C. in an electricfurnace, homogenized by stirring, and clarified. They were then castinto a metal mold that had been preheated to suitable temperature. Thecast glass was cooled to the glass transition temperature and thenimmediately placed in an annealing furnace, where it was graduallycooled to room temperature to obtain the optical glass.

The refractive index (n_(d)), Abbé number (ν_(d)), specific gravity,glass transition temperature, and liquidus temperature of the variousglasses obtained by the above method were measured by the followingmethods. The results are given in Table 1. Additionally, the λ₈₀, λ₇₀,and λ₅ of the optical glasses of examples 1 to 17 were measured by theabove-described methods; the results are given in Table 1.

(1) Refractive Index (n_(d)) and Abbé Number (ν_(d))

Measured for optical glasses obtained by cooling at a gradualtemperature reduction rate of 30° C./hour.

(2) Glass Transition Temperature (T_(g))

Measured at a rate of temperature increase of 4° C./minute with athermomechanical analyzer made by Rikagu Denki K.K.

(3) Specific Gravity

Calculated by Archimedes' method.

(4) Liquidus Temperature (L. T.)

Roughly 50-gram glass samples were charged to a platinum crucible andmelted for about 15 to 60 minutes at about 1,200 to 1,300° C. The glasssamples were then maintained for about 2 hours at 980° C., 990° C.,1000° C., 1010° C., 1020° C., 1030° C., 1040° C., 1050° C., 1060° C.,1070° C., 1080° C., 1090° C., or 1100° C., respectively; and cooledwhile observing with a microscope whether or not crystals precipitated.The lowest temperature at which crystals were not observed was adoptedas the liquidus temperature (L.T.).

TABLE 1 B₂O₃ SiO₂ Li₂O ZnO La₂O₃ Gd₂O₃ Y₂O₃ ZrO₂ Ex. 1 50.50 11.00 8.008.80 10.00 2.50 3.50 1.20 Ex. 2 50.50 11.00 7.50 8.80 10.00 2.50 3.501.20 Ex. 3 50.00 11.00 9.00 8.80 9.50 3.50 3.00 1.20 Ex. 4 48.06 11.659.22 10.00 9.32 3.01 3.40 0.97 Ex. 5 44.00 15.50 8.00 11.00 9.50 4.002.00 1.00 Ex. 6 44.50 15.00 8.00 11.00 8.50 5.80 2.00 0.20 Ex. 7 46.0014.50 8.00 11.00 8.50 5.50 2.00 0.00 Ex. 8 46.03 13.86 7.92 10.89 8.425.45 1.98 0.00 Ex. 9 51.00 7.00 4.50 12.50 10.50 7.00 1.00 4.00 Ex. 1051.00 7.00 4.50 10.00 10.50 7.00 1.00 4.00 Ex. 11 51.00 7.00 5.50 5.0010.50 7.00 1.00 4.00 Ex. 12 52.67 7.53 4.84 5.38 11.29 7.53 1.08 4.30Ex. 13 56.66 7.78 5.00 5.56 11.67 7.78 1.11 4.44 Ex. 14 61.14 7.25 4.665.18 10.88 8.81 1.04 1.04 Ex. 15 61.45 7.29 4.69 5.21 10.94 9.38 1.040.00 Ex. 16 50.29 12.29 5.03 5.59 12.85 6.70 2.23 2.23 Ex. 17 52.09 7.643.81 11.61 11.10 7.72 1.05 3.57 Ex. 18 51.41 8.04 4.00 11.67 11.44 7.911.18 4.11 Ex. 19 51.21 8.00 3.98 11.62 11.14 7.70 1.15 4.97 Ex. 20 52.088.07 4.02 9.96 11.49 7.94 1.19 4.13 Ex. 21 52.09 8.07 4.20 10.84 11.397.87 1.18 3.24 Ex. 22 52.08 8.18 4.10 10.83 11.43 7.90 1.18 3.24 Ex. 2352.56 8.07 4.02 11.73 11.24 7.77 1.16 1.41 Ex. 24 51.61 8.52 4.10 10.8111.51 7.96 1.19 3.24 Ex. 25 51.58 8.33 4.09 11.16 11.63 7.74 1.18 3.23Ex. 26 58.70 6.96 8.46 4.97 10.45 8.46 1.00 1.00 Ex. 27 58.50 8.36 6.965.01 10.68 8.54 1.02 0.93 Ex. 28 59.07 7.62 6.67 5.15 11.08 8.87 1.060.48 Ex. 29 59.98 7.50 6.56 5.06 11.03 8.82 1.05 0.00 Ex. 30 61.67 5.655.65 6.03 11.08 8.86 1.06 0.00 MgO CaO SrO BaO Ta₂O₅ Total Ex. 1 0.004.50 0.00 0.00 0.00 100.00 Ex. 2 0.00 5.00 0.00 0.00 0.00 100.00 Ex. 30.00 4.00 0.00 0.00 0.00 100.00 Ex. 4 0.00 4.37 0.00 0.00 0.00 100.00Ex. 5 0.00 5.00 0.00 0.00 0.00 100.00 Ex. 6 0.00 5.00 0.00 0.00 0.00100.00 Ex. 7 0.00 4.50 0.00 0.00 0.00 100.00 Ex. 8 0.00 5.45 0.00 0.000.00 100.00 Ex. 9 2.50 0.00 0.00 0.00 0.00 100.00 Ex. 10 5.00 0.00 0.000.00 0.00 100.00 Ex. 11 9.00 0.00 0.00 0.00 0.00 100.00 Ex. 12 5.38 0.000.00 0.00 0.00 100.00 Ex. 13 0.00 0.00 0.00 0.00 0.00 100.00 Ex. 14 0.000.00 0.00 0.00 0.00 100.00 Ex. 15 0.00 0.00 0.00 0.00 0.00 100.00 Ex. 160.00 2.79 0.00 0.00 0.00 100.00 Ex. 17 0.00 1.41 0.00 0.00 0.00 100.00Ex. 18 0.00 0.18 0.00 0.00 0.06 100.00 Ex. 19 0.00 0.18 0.00 0.00 0.05100.00 Ex. 20 0.00 1.06 0.00 0.00 0.06 100.00 Ex. 21 0.00 1.06 0.00 0.000.06 100.00 Ex. 22 0.00 1.06 0.00 0.00 0.00 100.00 Ex. 23 0.00 1.98 0.000.00 0.06 100.00 Ex. 24 0.00 1.06 0.00 0.00 0.00 100.00 Ex. 25 0.00 1.060.00 0.00 0.00 100.00 Ex. 26 0.00 0.00 0.00 0.00 0.00 100.00 Ex. 27 0.000.00 0.00 0.00 0.00 100.00 Ex. 28 0.00 0.00 0.00 0.00 0.00 100.00 Ex. 290.00 0.00 0.00 0.00 0.00 100.00 Ex. 30 0.00 0.00 0.00 0.00 0.00 100.00(ZrO₂ + Ta₂O₅)/ ZnO/(La₂O₃ + (CaO + SrO + BaO)/ (La₂O₃ + Gd₂O₃ + Y₂O₃)(La₂O₃ + Gd₂O₃ + Y₂O₃) Gd₂O₃ + Y₂O₃) Ex. 1 0.55 0.28 0.08 Ex. 2 0.550.31 0.08 Ex. 3 0.55 0.25 0.08 Ex. 4 0.64 0.28 0.06 Ex. 5 0.71 0.32 0.06Ex. 6 0.67 0.31 0.01 Ex. 7 0.69 0.28 0.00 Ex. 8 0.69 0.35 0.00 Ex. 90.68 0.00 0.22 Ex. 10 0.54 0.00 0.22 Ex. 11 0.27 0.00 0.22 Ex. 12 0.270.00 0.22 Ex. 13 0.27 0.00 0.22 Ex. 14 0.25 0.00 0.05 Ex. 15 0.24 0.000.00 Ex. 16 0.26 0.13 0.10 Ex. 17 0.58 0.07 0.18 Ex. 18 0.57 0.01 0.20Ex. 19 0.58 0.01 0.25 Ex. 20 0.48 0.05 0.20 Ex. 21 0.53 0.05 0.16 Ex. 220.53 0.052 0.16 Ex. 23 0.58 0.10 0.07 Ex. 24 0.53 0.05 0.16 Ex. 25 0.540.05 0.16 Ex. 26 0.25 0.00 0.05 Ex. 27 0.25 0.00 0.05 Ex. 28 0.25 0.000.02 Ex. 29 0.24 0.00 0.00 Ex. 30 0.29 0.00 0.00 Tg LT Specific λ 80 λ70 λ 5 nd νd (° C.) (° C.) gravity (nm) (nm) (nm) Ex. 1 1.71287 54.13583 990 3.88 363 340 253 Ex. 2 1.71335 54.00 586 990 3.88 361 339 254Ex. 3 1.71359 53.82 571 990 3.90 353 333 259 Ex. 4 1.71364 53.50 574 9903.90 356 334 260 Ex. 5 1.71664 53.36 574 1040 3.98 365 341 257 Ex. 61.71757 53.55 575 1020 4.06 356 334 261 Ex. 7 1.71314 53.90 573 10104.00 357 336 276 Ex. 8 1.71390 53.84 576 1010 4.01 355 335 260 Ex. 91.74679 51.33 602 1000 4.32 388 360 293 Ex. 10 1.74438 51.62 606 10104.29 389 365 310 Ex. 11 1.73892 52.26 606 1030 4.22 396 374 322 Ex. 121.74523 51.93 617 1030 4.29 384 364 311 Ex. 13 1.74263 52.37 624 10004.25 359 336 260 Ex. 14 1.72477 54.32 630 1020 4.19 358 338 276 Ex. 151.72387 54.51 630 1030 4.21 359 338 276 Ex. 16 1.74440 52.72 614 10604.29 364 342 277 Ex. 17 1.75101 51.47 609 1020 4.37 361 338 276 Ex. 181.75491 51.01 609 1030 4.41 — — — Ex. 19 1.75560 50.85 608 1020 4.39 — —— Ex. 20 1.75348 51.33 611 1040 4.39 — — — Ex. 21 1.75017 51.56 607 10304.37 — — — Ex. 22 1.74985 51.66 608 1040 4.37 — — — Ex. 23 1.74370 52.15606 1040 4.34 — — — Ex. 24 1.75047 51.65 607 1040 4.38 — — — Ex. 251.75062 51.60 612 1040 4.37 — — — Ex. 26 1.72375 53.97 598 1040 4.15 — —— Ex. 27 1.72393 54.27 610 1040 4.17 — — — Ex. 28 1.72709 54.18 609 10404.22 — — — Ex. 29 1.72346 54.48 614 1040 4.19 — — — Ex. 30 1.72443 54.43621 1040 4.19 — — —

Manufacturing of Preforms for Precision Press Molding

Glass melts corresponding to examples 1 to 30 that had been clarifiedand homogenized were caused to flow at a constant rate out of a platinumalloy pipe the temperature of which had been adjusted to within atemperature range at which the glass could stably flow withoutdevitrifying. The dripping or drop cut method was employed to separateglass melt gobs equal in weight to the target preforms. The glass meltgobs were received in a receiving mold having gas blowholes in thebottom thereof, and molded into preforms for precision press moldingwhile blowing gas through the blowholes to float the glass gobs. Theseparation interval of the glass melt was adjusted and set to obtainspherical preforms and oblate spheroidal preforms.

Manufacturing of Optical Elements (Aspherical Lenses)

Preforms obtained by the above-described method were precision pressmolded with the pressing device shown in FIG. 1 to obtain asphericallenses. Specifically, a preform was positioned between the lower mold 2and upper mold 1 of a pressing mold, a nitrogen atmosphere was providedwithin a quartz tube 11, and a heater (not shown) was turned on to heatthe interior of quartz tube 11. The temperature within the pressing moldwas set to a temperature at which the glass being molded exhibited aviscosity of 10⁸ to 10¹⁰ dPa·s and, while maintaining this temperature,a pressing rod 13 was dropped downward to press down on upper mold 1 andpress the preform that had been placed in the pressing mold. Thepressure applied during pressing was 8 MPa and the pressing time was 30s. Following pressing, the pressure was removed. The molded glassarticle that had been press molded was cooled to a temperature at whichthe viscosity of the glass was 10¹² dPa·s or greater while still incontact with lower mold 2 and upper mold 1, cooled rapidly to roomtemperature, and then removed from the pressing mold to obtain anaspherical lens. In FIG. 1, support member 10 supports lower mold 2 andsleeve mold 3. Support rod 9 supports upper mold 1, lower mold 2, sleevemold 3, and support member 10, as well as receiving the pressureimparted by pressing rod 13. A thermocouple 14 is inserted into theinterior of lower mold 2 to monitor the temperature within the pressingmold.

The above-described lens is suitable for use as a lens constituting animage pickup optical system. Objective lenses having an aperture numberof 0.85 for DVDs were fabricated with a suitable pressing mold andpreforms.

The present invention provides a high refractive index, low dispersionoptical glass suited to precision press molding. Preforms for precisionpress molding can be manufactured from the optical glass of the presentinvention. The present invention further provides an optical elementcomprised of high refractive index, low dispersion glass.

Although the present invention has been described in considerable detailwith regard to certain versions thereof, other versions are possible,and alterations, permutations and equivalents of the version shown willbecome apparent to those skilled in the art upon a reading of thespecification and study of the drawings. Also, the various features ofthe versions herein can be combined in various ways to provideadditional versions of the present invention. Furthermore, certainterminology has been used for the purposes of descriptive clarity, andnot to limit the present invention. Therefore, any appended claimsshould not be limited to the description of the preferred versionscontained herein and should include all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

Having now fully described this invention, it will be understood tothose of ordinary skill in the art that the methods of the presentinvention can be carried out with a wide and equivalent range ofconditions, formulations, and other parameters without departing fromthe scope of the invention or any embodiments thereof.

All patents and publications cited herein are hereby fully incorporatedby reference in their entirety. The citation of any publication is forits disclosure prior to the filing date and should not be construed asan admission that such publication is prior art or that the presentinvention is not entitled to antedate such publication by virtue ofprior invention.

1. An optical glass having a refractive index n_(d) of 1.70 or greaterand an Abbé number of 50 or greater, and, given as mole percentages,comprising: B₂O₃ 20 to 80 percent, SiO₂  0 to 30 percent, Li₂O  1 to 25percent; ZnO  0 to 20 percent, La₂O₃  4 to 30 percent, Gd₂O₃  1 to 25percent, Y₂O₃  0 to 20 percent, ZrO₂  0 to 5 percent, MgO  0 to 25percent, CaO  0 to 15 percent, SrO  0 to 10 percent,

with the combined quantity of the above components being 97 percent orgreater; the molar ratio of {ZnO/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.8 or lessand the molar ratio of {(CaO+SrO+Bao)/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.8 orless; Ta₂O₅ may be incorporated as an optional component, with the molarratio {(ZrO₂+Ta₂O₅)/(La₂O₃+Gd₂O₃+Y₂O₃)} being 0.4 or less.
 2. The glassof claim 1, wherein the combined quantity of Li₂O and ZnO ranges from 2to 30 percent, and the molar ratio of ZnO/Li₂O is equal to or less than6.
 3. The glass of claim 1, wherein the refractive index n_(d) and Abbénumber nu_(d) satisfy equation (1) below:n _(d)≧2.235−0.1×nu _(d)  (1)
 4. A preform for precision press moldingcomprised of the glass of claim
 1. 5. An optical element comprised ofthe glass of claim
 1. 6. A method of manufacturing a preform forprecision press molding comprising separating glass melt gobs from anoutflowing glass melt and forming the glass melt gobs into a preform forprecision press molding in a cooling step, wherein the preform forprecision press molding is formed from the glass of claim
 1. 7. A methodof manufacturing an optical element comprising heating and precisionpress molding a preform of claim 4 to produce an optical element.
 8. Themethod of claim 7, wherein the preform is introduced to a pressing mold,the pressing mold and preform are heated together, and then precisionpress molding is conducted.
 9. The method of claim 7, wherein thepreform is heated and introduced into a pressing mold that has beenpreheated and then the preform is precision press molded.
 10. A preformfor precision press molding comprised of the glass of claim
 2. 11. Apreform for precision press molding comprised of the glass of claim 3.12. An optical element comprised of the glass of claim
 2. 13. An opticalelement comprised of the glass of claim
 3. 14. A method of manufacturinga preform for precision press molding comprising separating glass meltgobs from an outflowing glass melt and forming the glass melt gobs intoa preform for precision press molding in a cooling step, wherein thepreform for precision press molding is formed from the glass of claim 2.15. A method of manufacturing a preform for precision press moldingcomprising separating glass melt gobs from an outflowing glass melt andforming the glass melt gobs into a preform for precision press moldingin a cooling step, wherein the preform for precision press molding isformed from the glass of claim
 3. 16. A method of manufacturing anoptical element comprising heating and precision press molding a preformof claim 10 to produce an optical element.
 17. A method of manufacturingan optical element comprising heating and precision press molding apreform of claim 11 to produce an optical element.
 18. A method ofmanufacturing an optical element comprising heating and precision pressmolding a preform for precision press molding a glass prepared by themethod of claim 6 to produce an optical element.
 19. A method ofmanufacturing an optical element comprising heating and precision pressmolding a preform by the method of claim 14 to produce an opticalelement.
 20. A method of manufacturing an optical element comprisingheating and precision press molding a preform prepared by the method ofclaim 15 to produce an optical element.